Radar level gauge systems are in wide use for measuring process variables of a product contained in a tank, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity along the probe of the electromagnetic signals.
In some applications of radar level gauge systems and other systems for determining the filling level of a product in a tank, it is important to be able to securely tell if the filling level has reached a certain critical level (high or low) or if the filling level is in the normal filling level range. For example, it may be required that a system is provided for issuing a signal to indicate when the filling level exceeds a preset limit to safely be able to prevent an overfill condition.
Systems producing alarms are conventionally realized as mechanical systems including a float member which is mechanically connected to a detector. An example of such a system is disclosed in U.S. Pat. No. 5,649,450. However, such mechanical systems are the subject of many problems. For example, the environment in the tanks is often relatively rough, making the movable mechanical parts likely to malfunction over time. Further, the floating member needs to be in contact with the fluid, which is disadvantageous since the surface is normally not calm in use. Accordingly, these mechanical systems have problems with robustness, accuracy and reliability.
To address the above problems associated with mechanical systems for providing an overfill alarm signal, U.S. Pat. No. 7,319,401 provides a system for continuously determining a filling level, and to produce an alarm signal if the filling level is determined to exceed a preset threshold value indicative of a high level and/or overfill condition.
However, the operator of the tank application may still, depending on the application and regulations and/or operator internal guidelines need to periodically perform proof tests to verify operation of the overfill alarm system. Such proof tests may require the operator to manually measure the filling level and to compare the manually measured filled level value with that given by the system according to U.S. Pat. No. 7,319,401.
Hence, although the system disclosed in U.S. Pat. No. 7,319,401 provides a considerably higher reliability than the previous mechanical overfill alarm systems, there may still be room for improvement in order to further reduce the time and effort spent on proof testing of the overfill alarm system.